Milling tools formed for hobbing of a workpiece are disclosed in EP2487000, EP2662173, and EP2537617.
EP2487000 discloses a milling tool formed for hobbing of a workpiece having cylindrical cogs, such as gear wheels or the like, or having racks. The milling tool comprises a tool body having a peripheral surface and two opposite ends between which a central rotation axis extends, the tool body being rotatable in a predetermined direction of rotation around the central rotation axis. The tool further comprises a number of spaced-apart cutting teeth in the form of cutting inserts protruding from the peripheral surface, each cutting tooth comprising a cutting edge and a chip surface. The cutting teeth are arranged along an imaginary helix concentric with the central rotation axis so as to engage the workpiece successively along the helix. The cutting teeth are all arranged at the same radial distance from the central rotation axis.
The milling tool disclosed in EP2487000 is a kind of milling tool whose tool length needs to be long enough to cover the chord of the workpiece, so that the cutting teeth of the tool can engage the workpiece smoothly as the workpiece rolls into engagement with the tool. Failure to meet this prerequisite, in particular for rough machining, results in large forces being exerted on the foremost cutting teeth upon engagement with the workpiece. Relatively short tools are therefore associated with high noise levels. Long tools, having several cutting teeth simultaneously engaged with the workpiece, are associated with lower noise levels but are on the other hand prone to vibrations. Moreover, the costs associated with such long tools are high, both due to large material costs and due to large service costs, for example for replacing worn cutting inserts, or, if the tool is a solid high speed steel tool, for sharpening of the cutting edges. The high service costs follow from the irregular wear on the cutting teeth of the tool. The cutting teeth of the foremost and the rearmost turns of the helix remove less material than the cutting teeth located in the middle section of the tool and are therefore less susceptible to wear. Upon replacement of worn cutting inserts, a large number of inserts need to be inspected for wear, also inserts that do not need to be replaced. Furthermore, the wear on the tool body is irregular, since the middle section, where the forces exerted on the cutting teeth during machining are larger, wears faster than the end sections. For this reason, the expensive tool body often needs to be replaced due to wear on the middle section, although the end sections are still functional.
EP2662173 and EP2537617 disclose milling tools for hobbing comprising a tool body formed from several detachably interconnected disc-shaped segments, wherein each segment comprises a number of cutting teeth. With such tools, it is possible to replace only the worn sections of the tool body and thereby prolong the life of parts of the milling tool. However, the problem with irregular wear on the cutting teeth within the tool or even within a segment persists. Short milling tools of the kind disclosed in EP2662173 and EP2537617 are also associated with the disadvantages discussed above, in particular for rough machining of a workpiece.